1. Field of the Invention
The present invention relates to a semiconductor storage device, and more particularly to an electrically rewritable phase-change large capacity memory device that nonvolatilely stores a resistance value determined according to a phase change between a crystal state and an amorphous state of a metal compound.
2. Description of the Related Art
One type of nonvolatile storage devices uses the crystal state and the amorphous state of the metal compound as storage information. The storage material as generally used is tellurium compound. A principle in which information is stored by a difference in reflectivity between those states has been widely used in an optical information storage medium such as a DVD (digital versatile disk).
In recent years, there has been proposed that the above principle is also used in electrical information storage. This is a method of detecting a difference in electric resistance between the amorphous material and the crystal, that is, a high resistive state of amorphous material and a low resistive state of crystal according to the amount of current or a change in voltage, differently from an optical technique. This is called “phase change memory”, and, for example, disclosed in Japanese Patent Application Laid-Open Publication No. 2003-100085 as a related art document. The present invention relates to the storage of electric information.
FIG. 1 shows a structure of a basic memory cell in a conventional phase change memory. In the structure, a storage element 001 (phase change material) is combined with a selective device 002. In the phase change memory, a current is supplied to the storage element 001 from the selective device 002 through a contact plug 003 to generate Joule heat 004 in the storage element 001. The Joule heat 004 puts the storage element 001 into a crystal state or an amorphous state to store and retain information. The rewrite of the phase change memory is performed by, when the storage element 001 is put into an electrically high resistive amorphous state, rapidly cooling the storage element 001 after a temperature of the storage element 001 becomes equal to or higher than a melting point thereof by supply of a large current. When the storage element 001 is put into an electrically low resistive crystal state, the rewrite of the phase change memory is performed by setting the temperature of the storage element 001 to a crystallization temperature lower than the melting temperature by limiting a current to be supplied. In general, the resistance value of the storage element is changed by double digits to triple digits according to the phase change. For that reason, the phase change memory is largely different in a read signal between the crystal state and the amorphous state to ease sense operation.
On the other hand, when data is read from the phase change memory element, for example, switching transistors are connected in series to the phase change memory element, and only a switching transistor connected to a selected read word line 005 turns on to form a current pathway. As a result, because a current flows in only the selected phase change memory, data can be read from the phase change memory. However, since one switching transistor exists in each cell, it is apparent that the cell area becomes larger.